1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device, and more particularly, to an in-plane switching (IPS) mode LCD device where an opening region formed in the pixel region is increased and a disclination region is minimized to maximize transmittance of light to the LCD panel.
2. Background of the Related Art
Flat panel displays are available in a variety of types. Among them, the LCD device has become a popular display device because of the advantageous characteristics that the LCD device offers, such as having a high contrast ratio, being thin, being lightweight, having features suitable for displaying gray level or moving pictures, and having low power consumption. Specifically, some of the LCD devices have been identified as the next generation display devices, for example, ultra thin displays for wall TVs and displays for notebook computers. In addition, the LCD device is used for a mobile display since a smaller size panel can be easily manufactured.
Furthermore, the LCD device has various modes depending on characteristics of liquid crystal and electrode structures. Some examples of the LCD devices include a twisted nematic (TN) mode LCD device, a multi-domain mode LCD device, an optically compensated birefringence (OCB) mode LCD device, an IPS mode LCD device, and a vertical alignment (VA) mode LCD device. In the TN mode LCD device, liquid crystal directors are arranged in a twisted angle of 90° and voltages are applied to control the liquid crystal directors. In the multi-domain mode LCD device, one pixel is divided into a plurality of domains and main viewing angles of the respective domains vary from one another to obtain a wide viewing angle. In the OCB mode LCD device, a compensation film is attached to an outer surface of a substrate to compensate phase variation of light depending on a progress direction of the light. In the IPS mode LCD device, two electrodes are formed on one substrate and liquid crystal directors are twisted in parallel with an alignment film. In the VA mode LCD device, liquid crystal molecules are vertically arranged on an alignment film using a negative liquid crystal and a vertical alignment film.
The IPS mode LCD device includes a color filter array substrate and a thin film transistor array substrate. The color filter array substrate is provided with a black matrix layer that prevents light leakage and R/G/B color filter layers for representing colors. The thin film transistor array substrate is provided with gate and data lines for defining unit pixels, the thin film transistors (i.e., switching elements) are formed at each intersection of the respective gate and data lines, and common and pixel electrodes are alternately arranged to generate transverse electric fields.
A related art IPS mode LCD device will be described with reference to the accompanying drawings. FIG. 1 is a plane view illustrating a related art IPS mode LCD device, FIG. 2 is a diagram illustrating transmittance of light in FIG. 1, FIG. 3 is a plane view illustrating another related art IPS mode LCD device, and FIG. 4 is a diagram illustrating transmittance of light in FIG. 3.
As shown in FIG. 1, a thin film transistor array substrate includes gate lines 12, data lines 15, thin film transistors, common lines 25, a plurality of common electrodes 24, and a plurality of pixel electrodes 17. The gate lines 12 vertically cross the data lines 15 to define pixel regions. The gate insulating films are interposed between the gate lines 12 and the data lines 15. Each of the thin film transistors is formed at each intersection of the respective gate and data lines 12 and 15. The thin film transistor includes a gate electrode 12a, a gate insulating film, a semiconductor layer 14 and source and drain electrodes 15a and 15b. The common lines 25 are formed in parallel with the gate lines 12. The common electrodes 24 are formed extending from the common lines 25 and formed in parallel with the data lines 15. Each pixel electrode 17 is connected to a drain electrode 15b and formed in parallel with each common electrode 24. The pixel electrodes 17 are arranged alternating with the common electrodes 24 in the horizontal direction.
In the IPS mode LCD device, the common electrode 24 and the pixel electrode 17 are formed on the same substrate to rotate liquid crystal molecules horizontally. Voltages are applied between the common and pixel electrodes to generate the transverse electric field E, so that the arrangement of the liquid crystal molecules is controlled. If the distance between the common electrode 24 and the pixel electrode 17 is long, the electric field becomes weak, whereas, if the distance between the common electrode 24 and the pixel electrode 17 is too short, the number of electrodes increases and an opening ratio deteriorates. Therefore, the distance between the two electrodes is important and care should be given while choosing a proper range. As shown FIG. 1, the area enclosed by the common electrode 24 within the pixel region is divided into two blocks 30, in which the each block 30 has a width D obtained by an optimized design rule. However, since the size of the pixel region depends on models of the LCD device, it is difficult to optimally arrange the blocks. For example, when the common electrodes are formed at both edges inside the pixel region and the pixel electrodes 17 are inserted between the common electrodes, then even numbers of blocks are needed in a horizontal direction. For this reason, it is difficult to optimally arrange the blocks.
Moreover, if the number of pixel regions in a high resolution model is relatively more than that of the pixel regions in the other model having the same sized panel, the size of the pixel region is small. Therefore, only two blocks may be formed in the pixel region.
When the pixel region has a width and a length of 28 μm×84 μm, the data line 15, the pixel electrode 17, two common electrodes 24 having a width of 4 μm are arranged in the pixel region. In this case, an opening region within the pixel region has a width of approximately 10 μm. Accordingly, an opening ratio is greatly reduced.
If voltages are applied to the related art IPS mode LCD device, the transmittance of light occurs as shown in FIG. 2. The transverse electric field is not formed at a portion where the common electrode 24 is extended from the common line 25, to promote the arrangement of the liquid crystal molecules in a desired direction. As a result, a disclination “region A” where no light transmits is generated.
Meanwhile, as shown in FIG. 3, the width of the pixel region increases and its length decreases to obtain a pixel region having a width and a length of 42 μm×42 μm. To obtain a super-IPS mode, a pixel electrode 117 and a common electrode 124 may be formed in a bent shape. In FIG. 3, a reference number 112 denotes gate lines, a reference number 115 denotes data lines, and a reference number 125 denotes common lines
The super-IPS mode means that a domain is divided into two sub-domains to arrange the liquid crystal molecules. Such two domains can minimize an inverted domain region. However, as shown in FIG. 4, a domain boundary occurs at portions where the pixel electrode 117 and the common electrode 124 are bent, so that the alignment direction of the liquid crystal molecules is divided. In this case, module efficiency is reduced, and the vertical electric field and the horizontal electric field (transverse electric field) are affected by each other to cause a wide disclination region (dark portion).
In other words, the disclination region where the liquid crystal molecules are not arranged in a desired direction corresponds to a region where the horizontal electric field is generated between each common electrode 124 and each pixel electrode 117. The generated horizontal electric field interferes with the vertical electric field generated either between adjacent common electrodes 124 or between the pixel electrodes 117 at corners of the pixel region. Since the light is not transmitted to the disclination region, mode efficiency is reduced.